AK: Rename Time to Duration

That's what this class really is; in fact that's what the first line of
the comment says it is.

This commit does not rename the main files, since those will contain
other time-related classes in a little bit.

Userland/Libraries/LibIPC/Encoder.cpp

@@ -85,7 +85,7 @@ ErrorOr<void> encode(Encoder& encoder, JsonValue const& value)
 }
 
 template<>
-ErrorOr<void> encode(Encoder& encoder, Time const& value)
+ErrorOr<void> encode(Encoder& encoder, Duration const& value)
 {
     return encoder.encode(value.to_nanoseconds());
 }